1. Field of the Invention
The present invention relates to a process for regenerating a filter which has been clogged and rendered inutile by solid carbonaceous materials contained in heavy oils or pitches derived from petroleum or coal, and, in particular, to a process for regenerating a filter, which has been clogged and rendered inutile by solid carbonaceous materials contained in heavy oils or pitches derived from petroleum or coal, comprising immersing the filter in an aqueous solution of hydrogen peroxide with heating, thus removing the solid carbonaceous materials clogging the filter. One preferred embodiment of the process of the present invention relates to a process for regenerating a filter, which has been clogged and rendered inutile by solid carbonaceous materials contained in heavy oils or pitches derived from petroleum or coal, comprising immersing the filter in an aqueous solution of hydrogen peroxide, heating it at a temperature between 50.degree.-100.degree. C., removing the aqueous solution of hydrogen peroxide, then, if required, further treating the filter by an ultrasonic wash, thus removing the solid carbonaceous materials clogging the filter.
2. Description of the Prior Art
Various types of carbon or graphite materials such as binder pitches, electrodes, graphite blocks, carbon fibers, and the like are manufactured from heavy oils such as residuum of the crude oil vacuum distillation unit, decant oil obtained in FCC unit, naphtha tar, i.e., bottom oil of ethylene plant, or the like; coal tars and the like produced in coke ovens; or pitches obtained from any of these, as raw materials. In the manufacture of these products, undesired components, such as components of an extremely high molecular weight contained in the raw material, free carbon, coke or extremely heavy components produced during the manufacturing process, as well as solid materials, are conventionally removed to ensure a higher quality of the target products. Also, it is very often conducted that as one of the manufacturing processes, an intermediate product obtained by removing the above-mentioned undesired components is subjected to a thermal polymerization process under specific conditions to recover only components polymerized by the thermal polymerization. In the removal of these undesired components from the raw material or from the intermediate product from the manufacturing process, or in the recovery only of the polymerized component in the product from the thermal polymerization process for the intermediate product from which the undesired components have been removed, the raw material, the intermediate product, or the thermally polymerized component contained in the intermediate product from which undesired component has been removed is generally dissolved in a suitable solvent or melted by heating to deposit such undesired components or polymerized components and to remove or recover them as undissolved or unmelted components. In this instance, these undissolved or unmelted components are removed or recovered by means of a filter. The major portion of the undissolved or unmelted components is made up of carbonaceous compounds consisting of carbon and hydrogen, even though it contains small amounts of metallic compounds, or nitrogen, sulfur, and silicon compounds, particularly in the case where they are derived from deposition of undesired components. In the present invention, the undissolved or unmelted components, both in the case where they are derived from deposition of undesirable components which are to be removed and in the case where they are deposited polymerized components which are the target material, are herein referred to as "solid carbonaceous materials". Also, in the present invention, the above-mentioned heavy oils such as the crude oil vacuum distillation residuum, FCC decant oil, naphtha tar, or the like; coal tars and the like produced in coke ovens; or pitches obtained from these heavy oils or coal tars, as well as intermediate products from the manufacturing process and the materials from the thermal polymerization process for the intermediate product from which the undesired components have been removed, are collectively referred to as "heavy oils or pitches derived from petroleum or coal".
Taking the manufacture of carbon fiber by spinning from molten pitch as a specific example, removing solid carbonaceous materials with a diameter of several microns from the pitch is essential. The reason is that they can be a cause of the fiber cutoff during the melt spinning of carbon fiber since carbon fiber has a diameter of less than ten and several microns and the existence of solid carbonaceous materials can also impair the characteristics of the carbon fiber. Normally, a filter with an aperture of several microns to several tens of microns is provided immediately before the spinning operation to remove the solid carbonaceous materials in the pitch, or a method is adopted by which the spinning apparatus is charged with pitch from which the solid carbonaceous materials have been removed by prior filtration. It is thus possible to manufacture a good carbon fiber in a stable manner by filtering the raw material immediately before the spinning nozzle or by a similar method.
Generally, materials such as metals, glass, polymers, papers, or the like are used as a filter medium. These filter media can be provided in many forms, such as a woven or non-woven fabric, or a porous member formed from sintered or bound particles. Among these, from the aspect of ensuring excellent resistance to corrosion, heat and solvents, a metal, particularly stainless steel, or glass is mainly used as the filter medium for removing solid carbonaceous materials contained in heavy oils or pitches derived from petroleum or coal. Stainless steel filters include stainless steel in the form of sheets of woven or non-woven fabric, a sintered metal formed by sintering minute particles of stainless steel, and the like. They are provided as sheets, cylinders, or units with a large filtration area formed by providing ribs in the axial direction of the cylinder. In addition, a unit made from layered fabrics with a number of fiber sheets having apertures with different sizes or a very thick filter medium can be utilized to improve filtering performance or resistance to filtering pressure.
In the removal of solid carbonaceous materials from heavy oils or pitches derived from petroleum or coal (hereinafter referred to as "heavy oils") by filtration, the solid carbonaceous materials build up on the surface of the filter medium, causing the filtering pressure to rise and reducing the rate of filtration. When the pressure goes up beyond a certain level or the rate of filtration falls below an acceptable range, the filter medium must be changed or regenerated to remove the solid carbonaceous materials built up on its surface. It is possible to scrape away the solid carbonaceous materials built up on the surface of the filter medium relatively easily, but because the filter medium itself has a certain thickness, there are many cases where the solid carbonaceous materials penetrate into the filter medium and cause clogging. In such a case the removal of these materials and the regeneration of the filter medium are not easily accomplished. Methods hitherto used to remove the solid carbonaceous materials which have penetrated into the filter medium and caused clogging include washing the filter with an organic solvent, for example, immersing it in quinoline which is reputed to most efficiently dissolve heavy oils, then ultrasonically washing it or washing it under heating; ultrasonically washing the filter with an alkaline washing agent, e.g., with Contaminon (a product of Wako Pure Chemical Industries, Ltd., a kind of non-ionic surfactant, pH: ca. 10), etc.; and the like. However, with these methods it is impossible to completely remove the solid carbonaceous materials from inside of the filter medium. Thus, regenerating the filter medium to restore a performance equivalent to a new filter medium is extremely difficult. When the filter medium is made of glass, it is known to be possible to completely remove the solid carbonaceous materials by immersing the filter medium in a solution containing chromic acid, but the effect on the environment of the chromium ions contained in the waste liquid is a problem. Significant cost is involved in disposing of this waste liquid. In addition, the regeneration with a chromic acid solution cannot be applicable to the treatment, for example, of pitch used to manufacture carbon fiber, since a metallic filter medium with a sufficient strength must be used for such a filtration.
If a filter medium which has not been adequately regenerated is reused, an increase in filtering pressure or a drop in the rate of filtration is occurred within a very short time. The filter exchange cycle must therefore be shortened, and the works involved in the exchange and regeneration of filter media are very troublesome. In many cases, the filter medium must be discarded only by the use of several times, and in extreme cases, the filter medium used for the manufacture of carbon fiber, for example, is discarded without a reuse, resulting in a great increase in production costs.